A sheet processing apparatus configured to count and discriminate a variety of paper sheets has been practically used. A sheet processing apparatus takes in sheets one by one from an input module, and conveys them to an optical detection unit. An optical detection unit detects light from a sheet, and obtains an image based on the detected light. A sheet processing apparatus determines denomination, authentication, and possibility of recirculation of a sheet, based on the obtained image. A sheet processing apparatus assorts and stacks a sheet, based on the results of determination.
An illuminator to emit light to a sheet is deteriorated in light intensity over time. When the light intensity of an illuminator is deteriorated, an image obtained by an optical detection unit may be darkened.
Therefore, an optical detection unit is provided with an illuminator, a detector, and a standard reflector. An illuminator emits light in a predetermined area (a detection surface). A detector detects light from a detection surface, and obtains an image. A standard reflector (a reference member) applies reference light to the detector.
When a sheet is present on a detection surface, light emitted from an illuminator is reflected on the sheet. The light reflected on the sheet enters a detector. The detector obtains an image of the sheet by the incident light.
When a sheet does not present on a detection surface, light emitted from an illuminator is reflected on a reference member. The light reflected on the reference member (a reference light) enters a detector. Therefore, the detector obtains a reference light.
An optical detection unit corrects the gain of a signal detected by a detector based on a reference light detected by a reference member. An optical detection unit obtains an image of a sheet by the detector, which corrects the signal gain. Therefore, an optical detection unit can obtain a stable image of a sheet.
However, when a sheet is processed, dust and stains may be adhered to a reference member. In this case, a reference light detected by a detector from a reference member may fluctuate. In other words, a signal may not be exactly corrected, and a stable image may not be obtained by the detector. To solve this problem, it is necessary to remove dust and stains from a reference member.
For example, Japanese Patent Document 1 discloses a method of removing stains by providing felt or the like at a position to contact a reference member when the reference member is taken out.